Water Economy and Metabolism of Two Estrildine Finches

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Abstract
Thirty-two domesticated zebra finches (Taeniopyqia castanotis) and 38 wild black-rumped waxbills (Estrilda troglodytes) were studied. Waxbills die in 3 days without water, but many zebra finches live for weeks. Waxbills consume water equal to 42% of body weight per day, but zebra finches average only 24%. A rise in air temperature from 20 to 40 C causes waxbills to increase water consumption dramatically, but zebra finches show a less marked response. Waxbills cannot survive on a drinking solution more concentrated than 0.15 M NaCl; but zebra finches can tolerate 0.2 M, and some which have been deprived of water for a prior time can tolerate 0.4 M NaCl. Both species can distinguish a 0.05 M NaCl solution from distilled water. Measurements of CO2 production, O2 uptake, and the passage ot dyed food through the digestive system indicate that zebra finches and waxbills reach a postabsorptive state within 3 hrs. Waxbills achieve a minimal rate of metabolism (2.6 ml CO2 per gm per hr) only around 38C, and the rate increases sharply on either side of this point. Zebra finches have the same basal rate extending over a ther-moneutral zone from 36 to 42 C. Zebra finches are better able to withstand extremes of cold and heat than waxbills. There is no difference between the metabolism of watered zebra finches and those deprived of water. Waxbills evaporate 12 mg H2O per gm per hr between 2 and 37 C, while watered zebra finches lose 8.6 mg H2O per gm per hr. Evaporation in both species increases precipitiously above 37 C. Fasting waxbills can produce no more than 1 mg of metabolic water per 1.5 mg evaporated. Normally watered zebra finches do no better, but those which have been without water 30 or more days can reduce evaporation sufficiently to bring the ratio of water produced to water evaporated to unity. The mechanism for reducing evaporative water loss may involve some alteration in ventilation of the lungs and air sacs or temperature changes with reduction of the saturation vapor pressure in the respiratory tract. The different water economies of the swamp-dwelling waxbill and the xerophilous zebra finch may be explained by the different environmental demands on the 2 populations. Xerophilous birds are adapted for survival on a minimum intake of water during dry periods and for quick reproduction after rain. Newton''s empirical law of cooling does not account adequately for the metabolic responses of waxbills and zebra finches and some other small birds and mammals which have high lower critical temperatures and regression lines in the zone of chemical regulation which extrapolate to points on the abscissa above normal body temperature.